Many small internal combustion engines are supplied with a combustible charge of air and fuel using a carburetor. A typical carburetor includes a body defining a liquid fuel chamber, an air and fuel mixing passage, and one or more fuel passages in communication between the fuel chamber and the air and fuel mixing passage. The fuel passages communicate with the mixing passage between an air inlet at an upstream end and an air and fuel mixture outlet at a downstream end. Typically, a choke valve is disposed in the air and fuel mixing passage near the upstream end to control a quantity of air flowing into the mixing passage during engine cold starting and warm up. A throttle valve is disposed in the air and fuel mixing passage near the downstream end to control a quantity or flow rate of the air and fuel charge flowing out of the mixing passage to the operating engine. In operation, a pressure differential causes liquid fuel to flow out of the fuel passages and into the air and fuel mixing passage where the fuel becomes mixed with air to create the air and fuel charge.
The carburetor creates and controls the combustible charge of air and fuel by controlling the flow of liquid fuel into the air flowing through the mixing passage, and by controlling the flow of air into the mixing passage and/or the air and fuel mixture flowing out of the mixing passage. More specifically, the carburetor may be manipulated to adjust an air to fuel (A/F) ratio in accord with varying engine requirements during engine startup, idle, steady-state operation, maximum power output, changes in load and altitude, and the like. In one example, the choke valve may be closed to such an extent that pulsating vacuum induced by reciprocating pistons in the engine will be greater (or at a larger magnitude of sub-atmospheric pressure) than when the choke valve is open and, thus, will supply a greater or larger quantity of fuel into the mixing passage for a richer A/F ratio. In another example, one or more valves in communication with the fuel passages may be adjusted to supply more, or less, liquid fuel.
Automotive and other fuel injected large engines often use oxygen sensors or Lambda probes exposed to exhaust gas to indicate A/F ratio over a wide range of operating conditions. But such sensors or probes and related hardware and software can be cost prohibitive for some engine applications and particularly small engines or applications without a storage battery for the ignition system.